In a class of conventional mass spectrometry methods, a combined field (comprising trapping and supplemental field components of different spatial form) is established in an ion trap, and the combined field is changed to resonantly excite stably trapped ions for detection. For example, U.S. Pat. No. 3,065,640 (issued Nov. 27, 1962) describes a three-dimensional quadrupole ion trap with reference to FIG. 1 thereof. It teaches application of DC voltage 2 V.sub.dc and AC voltage 2 V.sub.ac across the trap's end electrode 13 and ring electrode 11 to establish a quadrupole trapping field in the trap, application of a supplemental voltage (having DC component V.sub.g and AC component 2 V.sub..beta.) across the quadrupole trap's end electrodes 12 and 13 to establish a combined (trapping and supplemental) field in the trap, and changing the combined field by increasing one or both of simultaneously applied voltages V.sub.g and V.sub.dc to eject trapped ions from the trap through a hole 25 through end electrode 12 for detection at an external detector 26 (see col. 3, lines 13-18, and col. 9, lines 9-23, for example).
In a class of conventional mass spectrometry techniques known as "MS/MS" methods, ions (known as "parent ions") having mass-to-charge ratio (hereinafter denoted as "m/z") within a selected range are isolated in an ion trap. The stably trapped parent ions are then allowed or induced to dissociate (for example, by colliding with background gas molecules within the trap) to produce ions known as "daughter ions." The daughter ions are then ejected from the trap (typically by resonant ejection) and detected.
For example, U.S. Pat. No. 4,736,101, issued Apr. 5, 1988, discloses an MS/MS method in which ions (having m/z's within a predetermined range) are trapped within a three-dimensional quadrupole trapping field (established by applying a trapping voltage across the ring and end electrodes of a quadrupole ion trap). The trapping field is then scanned to eject unwanted parent ions (ions other than parent ions having a desired m/z) consecutively from the trap. The trapping field is then changed again to become capable of storing daughter ions of interest. The trapped parent ions are then induced to dissociate to produce daughter ions, and the daughter ions are ejected consecutively (sequentially by mass-to-charge ratio) from the trap for detection.
U.S. Pat. No. 4,736,101 teaches (at column 5, lines 16-42) establishment of a supplemental AC field (in addition to the trapping field) in the trap after the dissociation period, while the trapping voltage is scanned (or while the trapping voltage is held fixed and the frequency of the supplemental AC field is scanned). The frequency of the supplemental AC field matches one of the components of the frequency spectrum of trapped ion oscillation, so that the supplemental AC field resonantly ejects a sequence of stably trapped ions from the trap as the secular frequency of each trapped ion (in the changing combined field) comes to match the frequency of the supplemental AC field.
Similarly, U.S. Pat. No. 4,882,484 (issued Nov. 21, 1989) teaches resonant ejection of stably trapped ions from a three-dimensional quadrupole (or approximately quadrupole) trap by scanning a combined field (having RF trapping and supplemental AC field components) that has been established in an ion trap region, such as by scanning the supplemental field's frequency, or holding the supplemental field frequency fixed while scanning a parameter of the trapping field.
A disadvantage of conventional resonant excitation techniques (including resonant ejection methods) is that the changing parameters of the combined field must be carefully controlled during resonant excitation to avoid simultaneous ejection (loss of mass resolution) of different ion species and other undesirable interference effects. For example, consider a conventional resonant excitation method in which the frequency of the supplemental AC field component of the combined field is swept or scanned while other combined field parameters are held fixed. In this case, the peak-to-peak amplitude of the AC supplemental voltage signal which establishes the supplemental AC field component must be carefully controlled (it must be maintained at a particular amplitude, to allow ions of interest to establish a resonance condition), and the rate of change of the supplemental voltage signal's AC frequency must also be carefully controlled, to avoid unacceptable mass resolution decrease due to simultaneous ejection of different ion species.
Quadrupole Mass Spectrometry and its Applications, edited by P. H. Dawson, published by Elsevier, 1976, teaches at pp. 49-50 that if the frequency of the supplemental field applied during resonant excitation is close to, but different from the resonant frequency of a stably trapped ion, ions having different resonant frequency may be simultaneously excited for detection by the changing combined field. This is said to undesirably limit resolving power.
Another conventional technique for exciting stably trapped ions is known as "mass selective instability" excitation. Examples of this technique are described in above-referenced U.S. Pat. No. 4,882,484, and in European Patent Application Publication No. 350,159A (published Jan. 10, 1990).
In mass selective instability excitation, a trapping field (typically a quadrupole trapping field) is established in a trap region to stably trap ions, and one or more parameters of the trapping field are swept (or scanned) to cause trapped ions to become sequentially unstable. Each stably trapped ion is characterized by parameters which map to a location within a stability diagram determined by the trapping field. FIG. 2 is an example of a stability diagram for a quadrupole trapping field (FIG. 2 will be discussed in greater detail below). With reference to FIG. 2, ions having "a" and "q" parameters within the stability envelope (within the region bounded by the four curves labeled .beta..sub.r =0, .beta..sub.z =1, .beta..sub.r =1, and .beta..sub.z =0) can be stably trapped in the quadrupole trapping field (the parameters "e" and "m" in FIG. 2 denote charge and mass, respectively). In performing mass selective instability excitation, the changing trapping field causes ions to become unstable and ejects them by moving the "a" and/or "q" parameters of a sequence of stably trapped ions outside the stability diagram (from within the stability diagram).
In some conventional mass selective instability excitation methods, a supplemental field is established in the trap during sweeping or scanning of the trapping field (an AC oscillator is connected to one or both of the electrodes). For example, above-referenced European Patent Application 350,159A teaches (at column 3, line 58, through column 4, line 25) application of a supplemental AC voltage across the end electrodes of a quadrupole ion trap while sweeping or scanning parameters of a quadrupole trapping field in the trap. It also teaches that the quadrupole trapping field parameters can be fixed and the supplemental AC frequency swept or scanned to accomplish mass selective instability ejection. It also teaches that the quadrupole trapping field has an RF frequency component, the frequency of the supplemental AC voltage is preferably approximately equal to half the RF frequency (e.g., within plus or minus twenty percent of half the RF frequency), and the supplemental AC voltage has a frequency that matches the characteristic frequency of ion motion in the z (axial) direction.
In another conventional trapped ion excitation method, described in U.S. Pat. No. 4,749,860 (issued Jun. 7, 1988), a supplemental AC voltage is applied across end electrodes of a quadrupole ion trap while sweeping or scanning parameters of a quadrupole trapping field in the trap (for example, during period C in the scan diagram shown in FIG. 2 of U.S. Pat. No. 4,749,860). The frequency of the supplemental AC voltage is chosen to resonantly eject a sequence of stably trapped ions. At the same time, other trapped ions are said to become sequentially unstable in the presence of the changing combined field.
However, a disadvantage of mass selective instability excitation is that the dynamic range for the number of ions that can be stored and mass analyzed with sufficient mass resolution is very limited. Performance of the inventive method avoids this disadvantage by extending the dynamic range, and also avoids the above-described disadvantage of conventional resonant excitation methods.
In the context of quadrupole mass filter operation, rather than three-dimensional quadrupole trap operation, above-cited Quadrupole Mass Spectrometry and its Applications (1976), at pages 74-76, teaches superimposition of a sinusoidal supplemental field with the field establishing the quadrupole mass filter, with the frequency of the supplemental field differing by a small amount (.DELTA..omega.) from a secular frequency of a selected ion propagating through the filter. Application of such a supplemental field (which may be denoted as a "near resonance" or "off-resonance" supplemental field) is said to enable separation (filtering) of ions (by establishing a beat frequency condition) whose motion through the filter has such secular frequency from ions whose motion has slightly different secular frequency. However, this reference does not suggest that an "off-resonance" field should be applied as one component of a changing combined field (established in a three-dimensional ion trap) to sequentially excite selected trapped ions (by a mechanism other than resonant excitation or mass selective instability excitation). Also in the context of mass filter operation, rather than three-dimensional trap operation, U.S. Pat. No. 2,950,389, issued Aug. 23, 1960, teaches that application of a near resonance supplemental field enables separation (filtering) of ions whose motion through the filter has a first secular frequency from ions whose motion has slightly different secular frequency.